DOCSLIB.ORG

Crustal Structure and Relocated Earthquakes in the Puget Lowland, Washington, from High-Resolution Seismic Tomography T

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Crustal Structure and Relocated Earthquakes in the Puget Lowland, Washington, from High-Resolution Seismic Tomography T JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B12, 2381, doi:10.1029/2001JB000710, 2002 Crustal structure and relocated earthquakes in the Puget Lowland, Washington, from high-resolution seismic tomography T. M. Van Wagoner,1 R. S. Crosson, K. C. Creager, G. Medema, and L. Preston Department of Earth and Space Sciences, University of Washington, Seattle, Washington, USA N. P. Symons Sandia National Laboratories, Albuquerque, New Mexico, USA T. M. Brocher U.S. Geological Survey, Menlo Park, California, USA Received 13 June 2001; revised 19 July 2002; accepted 21 August 2002; published 31 December 2002. [1] The availability of regional earthquake data from the Pacific Northwest Seismograph Network (PNSN), together with active source data from the Seismic Hazards Investigation in Puget Sound (SHIPS) seismic experiments, has allowed us to construct a new high- resolution 3-D, P wave velocity model of the crust to a depth of about 30 km in the central Puget Lowland. In our method, earthquake hypocenters and velocity model are jointly coupled in a fully nonlinear tomographic inversion. Active source data constrain the upper 10–15 km of the model, and earthquakes constrain the deepest portion of the model. A number of sedimentary basins are imaged, including the previously unrecognized Muckleshoot basin, and the previously incompletely defined Possession and Sequim basins. Various features of the shallow crust are imaged in detail and their structural transitions to the mid and lower crust are revealed. These include the Tacoma basin and fault zone, the Seattle basin and fault zone, the Seattle and Port Ludlow velocity highs, the Port Townsend basin, the Kingston Arch, and the Crescent basement, which is arched beneath the Lowland from its surface exposure in the eastern Olympics. Strong lateral velocity gradients, consistent with the existence of previously inferred faults, are observed, bounding the southern Port Townsend basin, the western edge of the Seattle basin beneath Dabob Bay, and portions of the Port Ludlow velocity high and the Tacoma basin. Significant velocity gradients are not observed across the southern Whidbey Island fault, the Lofall fault, or along most of the inferred location of the Hood Canal fault. Using improved earthquake locations resulting from our inversion, we determined focal mechanisms for a number of the best recorded earthquakes in the data set, revealing a complex pattern of deformation dominated by general arc-parallel regional tectonic compression. Most earthquakes occur in the basement rocks inferred to be the lower Tertiary Crescent formation. The sedimentary basins and the eastern part of the Olympic subduction complex are largely devoid of earthquakes. Clear association of hypocenters and focal mechanisms with previously mapped or proposed faults is difficult; however, seismicity, structure, and focal mechanisms associated with the Seattle fault zone suggest a possible high-angle mode of deformation with the north side up. We suggest that this deformation may be driven by isostatic readjustment of the Seattle basin. INDEX TERMS: 7205 Seismology: Continental crust (1242); 7218 Seismology: Lithosphere and upper mantle; 7230 Seismology: Seismicity and seismotectonics; KEYWORDS: tomography, structure, Puget, earthquakes Citation: Van Wagoner, T. M., R. S. Crosson, K. C. Creager, G. Medema, L. Preston, N. P. Symons, and T. M. Brocher, Crustal structure and relocated earthquakes in the Puget Lowland, Washington, from high-resolution seismic tomography, J. Geophys. Res., 107(B12), 2381, doi:10.1029/2001JB000710, 2002. 1. Introduction 1Now at ChevronTexaco North America Upstream, New Orleans, [2] Although most damaging historical earthquakes in Louisiana, USA. the region have been within the Wadati-Benioff zone of Copyright 2002 by the American Geophysical Union. the Cascadia subduction zone (e.g., the 2001 magnitude 0148-0227/02/2001JB000710$09.00 6.8 Nisqually earthquake [Malone et al., 2001]), much ESE 22 - 1 ESE 22 - 2 VAN WAGONER ET AL.: CRUSTAL STRUCTURE AND EARTHQUAKES Figure 1. Schematic geologic map of northwestern Washington and southern British Columbia, showing the model region (heavy inner rectangle). Abbreviations for major cities: E—Everett; O— Olympia; S—Seattle; T—Tacoma; V—Victoria. Abbreviations for faults: CRBF—Coast Range boundary fault; DF—Doty fault; DDMF-Darrington—Devils Mountain fault; HCF—Hood Canal fault; HRF—Hurricane Ridge fault; LRF—Leech River fault; OF—Olympia fault; SCF—Straight Creek fault; SF—Seattle fault; SHZ—St. Helens zone; SJF—San Juan fault; SqF—Sequim fault; SWIF—Southern Whidbey Island fault; TF—Tacoma fault; WRF—White River fault. Abbreviations for oil test wells: DS—Dungeness Spit #1; MK—Mobil Kingston #1; P18—Pope-Talbot #18-1; SC—Silvana Community #12-1; SS—Socal Schroeder #1; SW—Socal Whidbey #1; WS—Washington State #1. Abbreviations for geologic, structural, and topographic features: BH—Blue Hills; BkH—Black Hills; DB—Dabob Bay; EB—Everett basin; HC—Hood Canal; KA—Kingston arch; LS—Lake Sammamish; LW—Lake Washington; M—Metchosin igneous complex; PTB—Port Townsend basin; PS—Puget Sound; SB— Seattle basin; SU—Seattle uplift; TB—Tacoma basin. Abbreviations for Cascade volcanoes: MSH—Mt. St. Helens; MR—Mt. Rainier; MB—Mt. Baker. Sources: Blakely et al. [2002]; Brocher et al. [2001]; Brocher and Ruebel [1998]; Gower et al. [1985]; Johnson et al. [1999]; Schruben et al. [1994]; Tabor and Cady [1978a]. recent geological and geophysical research in the Puget investigations [e.g., Brocher et al., 2001; Blakely et al., Lowland of western Washington State (Figure 1) has been 2002]. prompted by increasing concern about hazards from crustal [3] Lees and Crosson [1990] carried out the first 3-D earthquake sources. A postulated earthquake about 1100 tomography for the Puget Lowland region, but were limited years ago on the Seattle fault [Bucknam et al., 1992] and by relatively few earthquake observations and sparse station concern about other possible crustal faults [e.g., Johnson et spacing. Symons and Crosson [1997] developed new inver- al., 1996, 1999] in the Lowland region motivated the sion technology and applied this to an improved earthquake recent SHIPS active source experiments [Brocher et al., data set plus limited explosion data. In their study, shallow 1999, 2000], and analyses based on these and other resolution was again limited by lack of surface source to VAN WAGONER ET AL.: CRUSTAL STRUCTURE AND EARTHQUAKES ESE 22 - 3 Figure 2. Map of the Puget Lowland model area showing source/receiver locations used to construct the high resolution model. The map area is the heavy inner rectangle shown in Figure 1. Temporary Wet- SHIPS stations are shown as diamonds (à) and Wet-SHIPS air gun tracks are shown as thin line segments. Permanent PNSN seismograph stations are shown as black triangles. Station distributions for both the Dry-SHIPS and the 1991 experiment are shown as line segments and experimental sources are shown as white stars. Higher confidence 3-D relocated earthquake epicenters are shown as white circles (6), lower confidence events as white squares (5). Earthquake observations were recorded only by PNSN stations. surface receiver observations. Seismic reflection profiling our knowledge of the crustal structure in this region. In this has been interpreted for structural details in only the upper study, the simultaneous analysis of earthquake data and few kilometers at most of the crust [e.g., Johnson et al., active source observations has allowed us to develop a high 1994, 1996; Pratt et al., 1997]. More recently, a first arrival quality 3-D model to a depth of 30 km, considerably greater P wave tomography model for the upper 11 km of the crust than previously reported [e.g., Brocher et al., 2001], and to was constructed by Brocher et al. [2001] in the same region simultaneously relocate nearly 1000 regional earthquakes as our study using data from the initial SHIPS experiment. for tectonic interpretation. In these previous studies, the structural transition to mid- crustal depths has not been accurately imaged. Since the 1.1. Geologic Setting midcrust is likely to be the seat of important tectonic [5] The tectonics of the Pacific Northwest region are processes in this forearc region, potentially valuable struc- dominated by the oblique, northeastward subduction of tural information has been missing. the Juan de Fuca plate beneath western North America. [4] The availability of active source data from SHIPS as Subduction of oceanic lithosphere during the Tertiary has well as two earlier experiments [Miller et al., 1997; Parsons led to the formation of two major geologic provinces in the et al., 1999], together with high quality crustal earthquake region: the Coast Range province on the west, which data acquired for the last 20 years from the PNSN regional includes the Olympic subduction complex (OSC) plus the network (Figure 2), provides an ideal opportunity to refine Puget Lowland, and the Cascade province on the east ESE 22 - 4 VAN WAGONER ET AL.: CRUSTAL STRUCTURE AND EARTHQUAKES (Figure 1) [Brandon and Calderwood, 1990; Snavely and fication of faults with possible reverse offsets [e.g., Johnson Wells, 1996]. et al., 1994, 1999; Brocher et al., 2001] is consistent with [6] In the OSC, metamorphosed Tertiary sedimentary the focal mechanisms of crustal earthquakes [Ma et al., rocks from the accretionary prism have been uplifted since 1996]. Northward compression is most likely the result of the Miocene to form the Olympic mountains [Tabor and oblique subduction [Wang, 1996], possibly in conjunction Cady, 1978a; Brandon et al., 1998]. Within the eastern with northward migration of the Cascadia forearc [Brandon Olympics is the steeply dipping Crescent Formation—thick and Calderwood, 1990; Wells et al., 1998]. submarine and subaerial Paleocene to Eocene basalts which [10] Geologic identification of shallow faults is hampered may have formed by extrusion associated with extension by thick glacial sediments covering much of the Lowland along the margin of north America [Wells et al., 1984; and diffuse crustal seismicity does not reveal distinct Babcock et al., 1992].
Load more

Recommended publications
  • Geologic History of Siletzia, a Large Igneous Province in the Oregon And
    Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot Wells, R., Bukry, D., Friedman, R., Pyle, D., Duncan, R., Haeussler, P., & Wooden, J. (2014). Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot. Geosphere, 10 (4), 692-719. doi:10.1130/GES01018.1 10.1130/GES01018.1 Geological Society of America Version of Record http://cdss.library.oregonstate.edu/sa-termsofuse Downloaded from geosphere.gsapubs.org on September 10, 2014 Geologic history of Siletzia, a large igneous province in the Oregon and Washington Coast Range: Correlation to the geomagnetic polarity time scale and implications for a long-lived Yellowstone hotspot Ray Wells1, David Bukry1, Richard Friedman2, Doug Pyle3, Robert Duncan4, Peter Haeussler5, and Joe Wooden6 1U.S. Geological Survey, 345 Middlefi eld Road, Menlo Park, California 94025-3561, USA 2Pacifi c Centre for Isotopic and Geochemical Research, Department of Earth, Ocean and Atmospheric Sciences, 6339 Stores Road, University of British Columbia, Vancouver, BC V6T 1Z4, Canada 3Department of Geology and Geophysics, University of Hawaii at Manoa, 1680 East West Road, Honolulu, Hawaii 96822, USA 4College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Administration Building, Corvallis, Oregon 97331-5503, USA 5U.S. Geological Survey, 4210 University Drive, Anchorage, Alaska 99508-4626, USA 6School of Earth Sciences, Stanford University, 397 Panama Mall Mitchell Building 101, Stanford, California 94305-2210, USA ABSTRACT frames, the Yellowstone hotspot (YHS) is on southern Vancouver Island (Canada) to Rose- or near an inferred northeast-striking Kula- burg, Oregon (Fig.
    [Show full text]
  • Petrogenesis of Siletzia: the World’S Youngest Oceanic Plateau
    Results in Geochemistry 1 (2020) 100004 Contents lists available at ScienceDirect Results in Geochemistry journal homepage: www.elsevier.com/locate/ringeo Petrogenesis of Siletzia: The world’s youngest oceanic plateau T.Jake R. Ciborowski a,∗, Bethan A. Phillips b,1, Andrew C. Kerr b, Dan N. Barfod c, Darren F. Mark c a School of Environment and Technology, University of Brighton, Brighton BN2 4GJ, UK b School of Earth and Ocean Science, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK c Natural Environment Research Council Argon Isotope Facility, Scottish Universities Environmental Research Centre, East Kilbride G75 0QF, UK a r t i c l e i n f o a b s t r a c t Keywords: Siletzia is an accreted Palaeocene-Eocene Large Igneous Province, preserved in the northwest United States and Igneous petrology southern Vancouver Island. Although previous workers have suggested that components of Siletzia were formed Geochemistry in tectonic settings including back arc basins, island arcs and ocean islands, more recent work has presented Geochemical modelling evidence for parts of Siletzia to have formed in response to partial melting of a mantle plume. In this paper, we Mantle plumes integrate geochemical and geochronological data to investigate the petrogenetic evolution of the province. Oceanic plateau Large igneous provinces The major element geochemistry of the Siletzia lava flows is used to determine the compositions of the primary magmas of the province, as well as the conditions of mantle melting. These primary magmas are compositionally similar to modern Ocean Island and Mid-Ocean Ridge lavas. Geochemical modelling of these magmas indicates they predominantly evolved through fractional crystallisation of olivine, pyroxenes, plagioclase, spinel and ap- atite in shallow magma chambers, and experienced limited interaction with crustal components.
    [Show full text]
  • Summary of Sexual Abuse Claims in Chapter 11 Cases of Boy Scouts of America
    Summary of Sexual Abuse Claims in Chapter 11 Cases of Boy Scouts of America There are approximately 101,135sexual abuse claims filed. Of those claims, the Tort Claimants’ Committee estimates that there are approximately 83,807 unique claims if the amended and superseded and multiple claims filed on account of the same survivor are removed. The summary of sexual abuse claims below uses the set of 83,807 of claim for purposes of claims summary below.1 The Tort Claimants’ Committee has broken down the sexual abuse claims in various categories for the purpose of disclosing where and when the sexual abuse claims arose and the identity of certain of the parties that are implicated in the alleged sexual abuse. Attached hereto as Exhibit 1 is a chart that shows the sexual abuse claims broken down by the year in which they first arose. Please note that there approximately 10,500 claims did not provide a date for when the sexual abuse occurred. As a result, those claims have not been assigned a year in which the abuse first arose. Attached hereto as Exhibit 2 is a chart that shows the claims broken down by the state or jurisdiction in which they arose. Please note there are approximately 7,186 claims that did not provide a location of abuse. Those claims are reflected by YY or ZZ in the codes used to identify the applicable state or jurisdiction. Those claims have not been assigned a state or other jurisdiction. Attached hereto as Exhibit 3 is a chart that shows the claims broken down by the Local Council implicated in the sexual abuse.
    [Show full text]
  • Ter Tiar Y Triassic Jurassic Cret Aceous
    AGE PERIOD EPOCH AGE PICKS (Ma) (Stage) (Ma) Interpretive Summary of Tectonic Evolution, Blue Mts and PNW Region (B. Dorsey, 2/26/2007) PLEIST 1.8 PLIOC MESSINIAN 5.3 7.1 Post-15 Ma: Cascade arc volcanism, CW rotation, N-S shortening in Yakima etc fold belts. L TORTONIAN 10 11.2 SERRAVALLIAN 14.8 MIO- M LANGHIAN 16.4 ~ 15-?? Ma: Uplift of Wallowa Mts, delamination of dense pluton root (Hales et al., 2005). BURDIGALIAN 20 CENE E 20.5 ~ 17-15 Ma: Voluminous CRB flows followed by paired migration of YS and NB hotspots. AQUITANIAN 23.8 Models for the CRBs include mantle plume (e.g. Camp and Ross, 2004), L CHATTIAN 28.5 back-arc extension, and upper mantle convection (Humphreys et al., 2000). 30 OLIG. E RUPELIAN 33.7 ~ 38-16 Ma: John Day Formation distal backarc basin, east of Cascade arc. L PRIABONIAN 37.0 BARTONIAN 40 41.3 ~ 40 Ma: Initiation of Cascadia subduction zone and magmatic arc. M LUTETIAN ~ 45 Ma: Accretion of Siletzia oceanic lithosphere (green patch in figure). TERTIARY 50 49.0 ~ 60-38 Ma: Clarno and Challis arc magmatism, possible shallowing of subduction. EOCENE E YPRESIAN 54.8 THANETIAN L 57.9 SELANDIAN 60 Paleoc. 61.0 Late Cretaceous to Early Tertiary (~ 90-55 Ma): E DANIAN 65.0 MAASTRICHTIAN Eastward migration of Idaho Batholith, Laramide Orogeny, 70 71.3 Possible continuation of dextral transpression in the WISZ (?) Humphreys et al. (2000) CAMPANIAN 80 LATE 83.5 SANTONIAN 85.8 Giorgis et al. (2005) CONIACIAN 89.0 Cretaceous Sevier Orogeny 90 TURONIAN 93.5 (east of here): Large thrust belt and CENOMANIAN foreland basin migrate eastward onto 100 99.0 North American craton.
    [Show full text]
  • Fluid Transport and Storage in the Cascadia Forearc Inferred from Magnetotelluric Data
    Fluid Transport and Storage in the Cascadia Forearc inferred from Magnetotelluric Data Gary Egbert Oregon State University Bo Yang ( [email protected] ) Zhejiang University https://orcid.org/0000-0001-6790-9882 Paul A. Bedrosian USGS Denver https://orcid.org/0000-0002-6786-1038 Kerry Key Columbia University https://orcid.org/0000-0003-1169-6683 Dean Livelybrooks University of Oregon Adam Schultz Oregon State University Anna Kelbert United States Geological Survey Blake Parris University of Oregon Article Keywords: geodynamics, tectonics, subduction Posted Date: June 22nd, 2021 DOI: https://doi.org/10.21203/rs.3.rs-475649/v1 License: This work is licensed under a Creative Commons Attribution 4.0 International License. Read Full License Fluid Transport and Storage in the Cascadia Forearc inferred from Magnetotelluric Data G. Egbert1 ,B. Yang*2, P. Bedrosian3, K. Key4, D. Livelybrooks5, A. Schultz1,7, A. Kelbert6, B. Parris5 1 Oregon State University, Corvallis OR, USA, 2 Key Laboratory of Geoscience Big Data and Deep Resource of Zhejiang Province, School of Earth Sciences, Zhejiang University, Hangzhou, China, 3 USGS, Denver CO, USA 4 Lamont-Doherty Earth Observatory, Columbia University, Palisades NY, USA 5 University of Oregon, Eugene OR, USU 6 USGS, Golden CO, USA 7 Pacific Northwest National Laboratory, Richland, WA, USA June 17, 2021 1 1 Abstract 2 Subduction of hydrated oceanic lithosphere can carry water deep into the Earth, with im- 3 portant consequences for a range of tectonic and magmatic processes. Most fluid is released at 4 relatively shallow depths in the forearc where it is thought to play a critical role in controlling 5 mechanical properties and seismic behavior of the subduction megathrust.
    [Show full text]
  • Field-Trip Guide to the Vents, Dikes, Stratigraphy, and Structure of the Columbia River Basalt Group, Eastern Oregon and Southeastern Washington
    Field-Trip Guide to the Vents, Dikes, Stratigraphy, and Structure of the Columbia River Basalt Group, Eastern Oregon and Southeastern Washington Scientific Investigations Report 2017–5022–N U.S. Department of the Interior U.S. Geological Survey Cover. Palouse Falls, Washington. The Palouse River originates in Idaho and flows westward before it enters the Snake River near Lyons Ferry, Washington. About 10 kilometers north of this confluence, the river has eroded through the Wanapum Basalt and upper portion of the Grande Ronde Basalt to produce Palouse Falls, where the river drops 60 meters (198 feet) into the plunge pool below. The river’s course was created during the cataclysmic Missoula floods of the Pleistocene as ice dams along the Clark Fork River in Idaho periodically broke and reformed. These events released water from Glacial Lake Missoula, with the resulting floods into Washington creating the Channeled Scablands and Glacial Lake Lewis. Palouse Falls was created by headward erosion of these floodwaters as they spilled over the basalt into the Snake River. After the last of the floodwaters receded, the Palouse River began to follow the scabland channel it resides in today. Photograph by Stephen P. Reidel. Field-Trip Guide to the Vents, Dikes, Stratigraphy, and Structure of the Columbia River Basalt Group, Eastern Oregon and Southeastern Washington By Victor E. Camp, Stephen P. Reidel, Martin E. Ross, Richard J. Brown, and Stephen Self Scientific Investigations Report 2017–5022–N U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior RYAN K. ZINKE, Secretary U.S.
    [Show full text]
  • Interpretation of the Seattle Uplift, Washington, As a Passive-Roof Duplex by Thomas M
    Bulletin of the Seismological Society of America, Vol. 94, No. 4, pp. 1379–1401, August 2004 Interpretation of the Seattle Uplift, Washington, as a Passive-Roof Duplex by Thomas M. Brocher, Richard J. Blakely, and Ray E. Wells Abstract We interpret seismic lines and a wide variety of other geological and geophysical data to suggest that the Seattle uplift is a passive-roof duplex. A passive- roof duplex is bounded top and bottom by thrust faults with opposite senses of vergence that form a triangle zone at the leading edge of the advancing thrust sheet. In passive-roof duplexes the roof thrust slips only when the floor thrust ruptures. The Seattle fault is a south-dipping reverse fault forming the leading edge of the Seattle uplift, a 40-km-wide fold-and-thrust belt. The recently discovered, north-dipping Tacoma reverse fault is interpreted as a back thrust on the trailing edge of the belt, making the belt doubly vergent. Floor thrusts in the Seattle and Tacoma fault zones, imaged as discontinuous reflections, are interpreted as blind faults that flatten updip into bedding plane thrusts. Shallow monoclines in both the Seattle and Tacoma basins are interpreted to overlie the leading edges of thrust-bounded wedge tips advancing into the basins. Across the Seattle uplift, seismic lines image several shallow, short- wavelength folds exhibiting Quaternary or late Quaternary growth. From reflector truncation, several north-dipping thrust faults (splay thrusts) are inferred to core these shallow folds and to splay upward from a shallow roof thrust. Some of these shallow splay thrusts ruptured to the surface in the late Holocene.
    [Show full text]
  • Hanford Sitewide Probabilistic Seismic Hazard Analysis 2014
    Hanford Sitewide Probabilistic Seismic Hazard Analysis 2014 Contents 4.0 The Hanford Site Tectonic Setting ............................................................................................... 4.1 4.1 Tectonic Setting.................................................................................................................... 4.1 4.2 Contemporary Plate Motions and Tectonic Stress Regime .................................................. 4.11 4.3 Late Cenozoic and Quaternary History ................................................................................ 4.16 4.3.1 Post-CRB Regional Stratigraphy ............................................................................... 4.17 4.3.2 Summary of Late Miocene, Pliocene and Quaternary History .................................. 4.19 4.4 Seismicity in the Hanford Site Region ................................................................................. 4.21 4.4.1 Crustal Seismicity ..................................................................................................... 4.21 4.4.2 Cascadia Subduction Zone Seismicity ...................................................................... 4.26 4.5 References ............................................................................................................................ 4.28 4.i 2014 Hanford Sitewide Probabilistic Seismic Hazard Analysis Figures 4.1 Plate tectonic setting of the Hanford Site .................................................................................... 4.1 4.2 Areal extent
    [Show full text]
  • The Spatial and Temporal Evolution of the Portland and Tualatin Basins, Oregon, USA
    The Spatial and Temporal Evolution of the Portland and Tualatin Basins, Oregon, USA by Darby Patrick Scanlon A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology Thesis Committee: John Bershaw, Chair Ashley R. Streig Ray E. Wells Portland State University 2019 © 2019 Darby Patrick Scanlon ABSTRACT The Portland and Tualatin basins are part of the Puget-Willamette Lowland in the Cascadia forearc of Oregon and Washington. The Coast Range to the west has undergone Paleogene transtension and Neogene transpression, which is reflected in basin stratigraphy. To better understand the tectonic evolution of the region, I modeled three key stratigraphic horizons and their associated depocenters (areas of maximum sediment accumulation) through space and time using well log, seismic, outcrop, aeromagnetic, and gravity data. Three isochore maps were created to constrain the location of Portland and Tualatin basin depocenters during 1) Pleistocene to mid-Miocene (0-15 Ma), 2) eruption of the Columbia River Basalt Group (CRBG, 15.5-16.5 Ma), and 3) Mid- Miocene to late Eocene time (~17-35 Ma). Results show that the two basins each have distinct mid-Miocene to Pleistocene depocenters. The depth to CRBG in the Portland basin reaches a maximum of ~1,640 ft, 160 ft deeper than the Tualatin basin. Although the Portland basin is separated from the Tualatin basin by the Portland Hills, inversion of gravity data suggests that the two were connected as one continuous basin prior to CRBG deposition. Local thickening of CRBG flows over a gravity low coincident with the Portland Hills suggests that Neogene transpression in the forearc reactivated the Sylvan- Oatfield and Portland Hills faults as high angle reverse faults.
    [Show full text]
  • Eocene Initiation of the Cascadia Subduction Zone: a Second Example of Plume-Induced Subduction Initiation? GEOSPHERE, V
    Research Paper THEMED ISSUE: Subduction Top to Bottom 2 GEOSPHERE Eocene initiation of the Cascadia subduction zone: A second example of plume-induced subduction initiation? GEOSPHERE, v. 15, no. 3 Robert J. Stern1 and Trevor A. Dumitru2 1Department of Geosciences, University of Texas at Dallas, Richardson, Texas 75080, USA https://doi.org/10.1130/GES02050.1 2Jasper Canyon Research, Inc., 4100 Campana Drive, Palo Alto, California 94306, USA 12 figures; 1 table; 1 supplemental file ■ ABSTRACT focus on how the new Cascadia subduction zone in the Pacific Northwest of CORRESPONDENCE: [email protected] North America (United States–Canada) formed at ca. 55 Ma. The existing paradigm for the major ca. 56–48 Ma subduction zone reorga- There are two principal ways that a new subduction zone may form: it may CITATION: Stern, R.J., and Dumitru, T.A., 2019, Eo- cene initiation of the Cascadia subduction zone: A sec- nization in the Pacific Northwest of North America is that: (1) the Siletzia large be induced or it may form spontaneously (Fig. 1). Induced nucleation of a sub- ond example of plume-induced subduction initiation?: igneous province erupted offshore to the west of North America, forming an duction zone (INSZ) is expected if plate convergence continues after a collision Geosphere, v. 15, no. 3, p. 659–681, https://doi.org oceanic plateau; (2) Siletzia then collided with North America, clogging the event arrests convergence in a preexisting subduction zone. There are two vari- /10.1130 /GES02050.1. Pacific Northwest segment of the Cordilleran subduction zone; and (3) the eties of INSZ, polarity reversal and transference.
    [Show full text]
  • Geological Survey Research 1962
    Geological Survey Research 1962 Synopsis of Geologic, Hydrologic, and Topographic Results GEOLOGICAL SURVEY PROFESSIONAL PAPER 450-A Geological Survey Research 1962 THOMAS B. NOLAN, Director GEOLOGICAL SURVEY PROFESSIONAL PAPER 450 Asynopsis of results ofgeologic, hydro logic, and topo­ graphic investigations for fiscalyear 1962, accom­ panied by short papers in the fields of geology, hydrology, topography, and allied sciences. Pub­ lished separately as Chapters A, B, C, D, and E UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1962 FOREWORD The reception accorded the 1960 and 1961 Annual Reviews of Geological Survey research has encouraged us to prepare this volume, "Geological Survey Research, 1962," in a con­ tinuing effort to publish more quickly the significant results of our current investigations. We continue to consider these reports as experimental and have again this year modified the content, format, and frequency of release of chapters in an attempt to serve better the interests of the users of the reports. The comments and suggestions of these users are here solicited and will be considered carefully as future volumes are planned. The current Annual Review consists of five chapters (Chapters A through E) of Pro­ fessional Paper 450. As in the preceding two Annual Reviews, Chapter A is a synopsis of recent findings in the many and varied lines of study pursued by Survey personnel. Chap­ ters, B, C, D, and E of this volume are collections of short articles in geology, hydrology, topography, and allied fields. These articles are numbered as follows: Prof. Paper 450-B Articles 1-59 Prof. Paper 450-C Articles 60-119 Prof.
    [Show full text]
  • Recovering America's Wildlife
    Business and Conservation Interests Support Dedicated Funding to Recover America’s Fish and Wildlife We, the undersigned, support preventing fish, wildlife , and plants from becoming endangered by cre- ating a dedicated federal fund for proactive conservation efforts, led by the states, territories, and tribal nations, to address the nation’s looming wildlife crisis. We support this concept as initially rec- ommended by the Blue Ribbon Panel on Sustaining America’s Diverse Fish and Wildlife Resources, comprised of national business and conservation leaders. Our nation has been blessed with a diverse array of flora and fauna. While some of these species are thriving, many more are facing increasing challenges and are in steep decline – increasing their pos- sibility of becoming endangered. State fish and wildlife agencies have identified 12,000 species na- tionwide in need of proactive conservation action. At the request of Congress, every state has developed a State Wildlife Action Plan to assess the health of their state’s fish and wildlife and outline conservation actions necessary to sustain them. Collec- tively, these action plans have identified these 12,000 species and formed a nationwide strategy to prevent them from becoming endangered. However, the current federal State and Tribal Wildlife Grants program is funded at only a fraction of what states need to conserve these species. State fish and wildlife biologists estimate that it would cost $1.3 billion annually to implement 75 percent of these actions. In addition, our nation’s tribes need similar funds ($97.5 million/annually) to address the hundreds of species and millions of acres of lands they steward.
    [Show full text]